Workaround TSan false positives in suspend/resume_thread

Workaround TSan false positives in suspend/resume_thread

* pthread_stop_world.c [!GC_OPENBSD_UTHREADS && !NACL
&& GC_ENABLE_SUSPEND_THREAD] (set_suspended_ext_flag): New static
function (with GC_ATTR_NO_SANITIZE_THREAD attribute); add comment.
* pthread_stop_world.c [!GC_OPENBSD_UTHREADS && !NACL
&& GC_ENABLE_SUSPEND_THREAD] (GC_suspend_thread): Call
set_suspended_ext_flag() instead of t->flags|=SUSPENDED_EXT.
* pthread_stop_world.c [!GC_OPENBSD_UTHREADS && !NACL
&& GC_ENABLE_SUSPEND_THREAD] (GC_resume_thread): Add
GC_ATTR_NO_SANITIZE_THREAD attribute; add comment.